U.S. patent application number 15/410237 was filed with the patent office on 2017-07-27 for optical transmission apparatus, optical transmission system, and method of controlling output of optical signal.
This patent application is currently assigned to FUJITSU LIMITED. The applicant listed for this patent is FUJITSU LIMITED. Invention is credited to Shoichiro Oda, Tomohiro YAMAUCHI.
Application Number | 20170214461 15/410237 |
Document ID | / |
Family ID | 57838193 |
Filed Date | 2017-07-27 |
United States Patent
Application |
20170214461 |
Kind Code |
A1 |
YAMAUCHI; Tomohiro ; et
al. |
July 27, 2017 |
OPTICAL TRANSMISSION APPARATUS, OPTICAL TRANSMISSION SYSTEM, AND
METHOD OF CONTROLLING OUTPUT OF OPTICAL SIGNAL
Abstract
An optical transmission apparatus includes a variable attenuator
configured to adjust output intensity of each wavelength signal
included in a multiplexed optical signal having been input; a
monitor configured to measure an output spectrum of the variable
attenuator; a calculation unit configured to calculate an amount of
spectral narrowing and an amount of spectral surplus, based on a
measured value by the monitor, and a target value set in advance;
and a control unit configured to control an amount of attenuation
of the variable attenuator, based on the amount of spectral
narrowing and the amount of spectral surplus.
Inventors: |
YAMAUCHI; Tomohiro;
(Kawasaki, JP) ; Oda; Shoichiro; (Fuchu,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FUJITSU LIMITED |
Kawasaki-shi |
|
JP |
|
|
Assignee: |
FUJITSU LIMITED
Kawasaki-shi
JP
|
Family ID: |
57838193 |
Appl. No.: |
15/410237 |
Filed: |
January 19, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 10/07957 20130101;
H04B 10/293 20130101; H04B 10/564 20130101; H04B 10/07955 20130101;
H04B 10/506 20130101; H04J 14/0256 20130101; H04J 14/0227 20130101;
H04J 14/0257 20130101; H04B 10/0793 20130101; H04B 10/0797
20130101; H04J 14/0221 20130101 |
International
Class: |
H04B 10/079 20060101
H04B010/079; H04J 14/02 20060101 H04J014/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 25, 2016 |
JP |
2016-011560 |
Claims
1. An optical transmission apparatus, comprising: a variable
attenuator configured to adjust output intensity of each wavelength
signal included in a multiplexed optical signal having been input;
a monitor configured to measure an output spectrum of the variable
attenuator; a calculation unit configured to calculate an amount of
spectral narrowing and an amount of spectral surplus, based on a
measured value by the monitor, and a target value set in advance;
and a control unit configured to control an amount of attenuation
of the variable attenuator, based on the amount of spectral
narrowing and the amount of spectral surplus.
2. The optical transmission apparatus as claimed in claim 1,
wherein the control unit controls the amount of attenuation of the
variable attenuator so that the amount of spectral narrowing
becomes minimum within a predetermined range of the amount of
spectral surplus, for each variable attenuation band as a unit of
controlling the variable attenuator.
3. The optical transmission apparatus as claimed in claim 1,
wherein the control unit controls the amount of attenuation of the
variable attenuator so that in a case where two adjacent components
of the wavelength signals are included in a variable attenuation
band as a unit of controlling the variable attenuator, spectral
surplus within a predetermined range is generated for each of the
two adjacent components of the wavelength signals.
4. The optical transmission apparatus as claimed in claim 1,
further comprising: a memory configured to store an upper limit
value of the amount of spectral surplus, wherein the control unit
controls the amount of attenuation of the variable attenuator so
that the amount of spectral narrowing becomes minimum within a
range in which the amount of spectral surplus does not exceed the
upper limit value.
5. The optical transmission apparatus as claimed in claim 4,
wherein the control unit controls the amount of attenuation of the
variable attenuator so that in a case where the calculated amount
of spectral surplus exceeds the upper limit value, the amount of
spectral surplus becomes the upper limit value.
6. The optical transmission apparatus as claimed in claim 4,
wherein the upper limit value is a value depending on reception
quality at a node to which the multiplexed optical signal is
output.
7. The optical transmission apparatus as claimed in claim 4,
further comprising: a transmitter configured to transmit a test
signal; a receiver configured to receive the test signal from the
transmitter in the optical transmission apparatus; and a
determination unit configured to determine the upper limit value
based on signal quality of the test signal measured by the receiver
and the amount of spectral surplus calculated by the calculation
unit.
8. The optical transmission apparatus as claimed in claim 4,
further comprising: a receiver configured to receive a test signal
from another optical transmission apparatus; and a determination
unit configured to determine the upper limit value based on signal
quality of the test signal measured by the receiver and the amount
of spectral surplus calculated by the calculation unit.
9. An optical transmission system, comprising: the optical
transmission apparatus as claimed in claim 1; and a network control
apparatus configured to control one or more of the optical
transmission apparatuses.
10. An optical transmission system, comprising: one or more optical
transmission apparatuses; and a network control apparatus
configured to control the optical transmission apparatuses, wherein
the optical transmission apparatus includes a variable attenuator
configured to adjust output intensity of each wavelength signal
included in a multiplexed optical signal having been input; a
monitor configured to measure an output spectrum of the variable
attenuator, based on a control signal from the network control
apparatus; a calculation unit configured to calculate an amount of
spectral narrowing and an amount of spectral surplus, based on a
measurement result by the monitor, and a target value set in
advance; and a control unit configured to control an amount of
attenuation of the variable attenuator based on the amount of
spectral narrowing and the amount of spectral surplus.
11. The optical transmission system as claimed in claim 10, further
comprising: an optical node not having a spectral monitor and
placed at a succeeding stage of the optical transmission apparatus;
and an optical transmission line connecting the optical
transmission apparatus with the optical node, wherein the optical
transmission apparatus outputs the multiplexed optical signal whose
output intensity is controlled so that the amount of spectral
surplus becomes a permissible upper limit value, and the amount of
spectral narrowing becomes minimum, to the optical transmission
line.
12. The optical transmission system as claimed in claim 11, further
comprising: a second optical transmission apparatus placed at a
succeeding stage of the optical node, wherein the second optical
transmission apparatus controls the output intensity of the
multiplexed optical signal so that the amount of spectral surplus
becomes the permissible upper limit value in a case where the
amount of spectral surplus calculated by the second optical
transmission apparatus exceeds the upper limit value.
13. A method of controlling output of an optical signal, the method
comprising: measuring a spectrum of a multiplexed optical signal
passing through a variable attenuator in an optical transmission
apparatus; calculating an amount of spectral narrowing and an
amount of spectral surplus based on a measured value of the
spectrum and a target value set in advance in the optical
transmission apparatus; and adjusting an amount of attenuation of
the variable attenuator based on the amount of spectral narrowing
and the amount of spectral surplus in the optical transmission
apparatus.
14. The method of controlling output of the optical signal as
claimed in claim 13, wherein the adjusting controls the amount of
attenuation of the variable attenuator so that the amount of
spectral narrowing becomes minimum within a predetermined range of
the amount of spectral surplus, for each variable attenuation band
as a unit of controlling the variable attenuator.
15. The method of controlling output of the optical signal as
claimed in claim 13, wherein the adjusting controls the amount of
attenuation of the variable attenuator so that in a case where two
adjacent components of the wavelength signals are included in a
variable attenuation band as a unit of controlling the variable
attenuator, spectral surplus within a predetermined range is
generated for each of the two adjacent components of the wavelength
signals.
16. The method of controlling output of the optical signal as
claimed in claim 13, the method further comprising: storing the
upper limit value of the amount of spectral surplus in a memory,
wherein the adjusting controls the amount of attenuation of the
variable attenuator so that the amount of spectral narrowing
becomes minimum within a range in which the amount of spectral
surplus does not exceed the upper limit value.
17. The method of controlling output of the optical signal as
claimed in claim 16, wherein the adjusting controls the amount of
attenuation of the variable attenuator so that in a case where the
calculated amount of spectral surplus exceeds the upper limit
value, the amount of spectral surplus becomes the upper limit
value.
18. The method of controlling output of the optical signal as
claimed in claim 16, the method further comprising: determining the
upper limit value depending on reception quality at a node to which
the multiplexed optical signal is output.
19. The method of controlling output of the optical signal as
claimed in claim 16, the method further comprising: transmitting a
test signal from a transmitter in the optical transmission
apparatus, to a receiver in the optical transmission apparatus; and
determining the upper limit value based on signal quality of the
test signal measured by the receiver and the amount of spectral
surplus calculated by the calculating.
20. The method of controlling output of the optical signal as
claimed in claim 16, the method further comprising: receiving a
test signal from another optical transmission apparatus by a
receiver in the optical transmission apparatus; and determining the
upper limit value based on signal quality of the test signal
measured by the receiver and the amount of spectral surplus
calculated by the calculating.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority of the prior Japanese Priority Application No. 2016-011560
filed on Jan. 25, 2016, the entire contents of which are hereby
incorporated by reference.
FIELD
[0002] The present disclosure relates to an optical transmission
apparatus, an optical transmission system, and a method of
controlling output of an optical signal.
BACKGROUND
[0003] To cope with ever increasing network traffic, high-density
optical multiplexing technologies have been researched such as
coherent optical orthogonal frequency division multiplexing
(CO-OFDM) and Nyquist wavelength division multiplexing (WDM).
CO-OFDM is a technology that places multiple optical signals with
fine frequency intervals on the frequency axis by using
orthogonality between signals. Nyquist WDM is a method of
high-density wavelength division multiplexing for optical signals
where the band is limited to the symbol rate frequency. Both
communication methods are highly efficient in terms of use of
frequencies or wavelengths.
[0004] A method of transmitting a signal in which multiple
subcarrier signals multiplexed or densely placed as a single signal
is also called "super-channel transmission". Collectively
transmitting multiple subcarrier signals realizes flexible,
large-capacity optical communication. A method has been proposed
that controls in advance (pre-emphasizes) the output power level to
the transmission line for each subcarrier, to reduce crosstalk
between the subcarriers, when executing super-channel transmission
(see, for example, Patent Document 1). Also, a method has been
proposed that sets the amount of attenuation of a subcarrier signal
in an edge part of the band of a super-channel signal to be
smaller, and sets the amount of attenuation of a subcarrier signal
in a center part of the band of the super-channel signal to be
greater, to prevent degradation of optical transmission quality
(see, for example, Patent Document 2).
[0005] Further, a technology has been known that detects the
wavelength of an optical signal by using an optical channel
monitor, and corrects a shift between a center wavelength of the
optical signal input into a wavelength selective switch, and a
center wavelength of a filter transmission band of the wavelength
selective switch (see, for example, Patent Document 3).
RELATED-ART DOCUMENTS
Patent Documents
[0006] [Patent Document 1] US Laid-open Patent Publication No.
2014/0314416
[0007] [Patent Document 2] Japanese Laid-open Patent Publication
No. 2013-106328
[0008] [Patent Document 3] Japanese Laid-open Patent Publication
No. 2014-116642
[0009] When multiplexing multiple subcarrier signals and pulse
signals to be transmitted as a single optical signal, controlling
the output level to an optical transmission line for each
subcarrier signal or pulse signal can prevent degradation of signal
quality on the transmission line.
[0010] However, a variable attenuator such as a wavelength
selective switch has a limited wavelength granularity for
controlling the amount of attenuation. In an optical signal in
which multiple subcarrier signals and pulse signals are densely
multiplexed, a single control slot (a wavelength band) may include
two adjacent signal components. Since a single target value is
generally used in a single control slot of a variable attenuator,
the amount of attenuation for each signal becomes deficient or
excessive in the wavelength band in which the two signal components
are included. The amount of attenuation becoming either deficient
or excessive degrades the signal quality.
SUMMARY
[0011] According to an aspect in the present disclosure, an optical
transmission apparatus includes a variable attenuator configured to
adjust output intensity of each wavelength signal included in a
multiplexed optical signal having been input; a monitor configured
to measure an output spectrum of the variable attenuator; a
calculation unit configured to calculate an amount of spectral
narrowing and an amount of spectral surplus, based on a measured
value by the monitor, and a target value set in advance; and a
control unit configured to control an amount of attenuation of the
variable attenuator, based on the amount of spectral narrowing and
the amount of spectral surplus.
[0012] The object and advantages of the embodiment will be realized
and attained by means of the elements and combinations particularly
pointed out in the claims. It is to be understood that both the
foregoing general description and the following detailed
description are exemplary and explanatory and are not restrictive
of the invention as claimed.
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIGS. 1A-1B are diagrams illustrating a technical problem to
be solved in optical transmission;
[0014] FIG. 2 is a diagram illustrating a basic concept to solve
the technical problem;
[0015] FIG. 3 is a schematic view of an optical transmission system
according to a first embodiment;
[0016] FIG. 4 is a basic configuration diagram of an optical
transmission apparatus according to the first embodiment;
[0017] FIGS. 5A-5B are diagrams illustrating spectral narrowing and
surplus;
[0018] FIGS. 6A-6B are diagrams illustrating a basic concept of
controlling output of an optical signal according to the first
embodiment;
[0019] FIG. 7A is a flowchart of a method of controlling output of
an optical signal;
[0020] FIG. 7B is a flowchart of a method of controlling output of
an optical signal;
[0021] FIG. 8 is a schematic view of an optical transmission system
that includes a node not having a spectral monitor according to a
second embodiment;
[0022] FIG. 9 is a diagram illustrating an example of a
configuration of nodes in a case where a node not having a spectral
monitor is included;
[0023] FIG. 10 is a diagram illustrating Control (1) in a case
where a node not having a spectral monitor is included;
[0024] FIG. 11 is a diagram illustrating Control (1) in a case
where a node not having a spectral monitor is included;
[0025] FIG. 12 is a diagram illustrating another example of a
configuration of nodes in a case where a node not having a spectral
monitor is included;
[0026] FIG. 13 is a diagram illustrating Control (2) in a case
where a node not having a spectral monitor is included;
[0027] FIG. 14 is a flowchart illustrating a control method
according to the second embodiment;
[0028] FIGS. 15A-15B are diagrams illustrating determination of an
upper limit value of the amount of spectral surplus;
[0029] FIG. 16 is a diagram illustrating a method of describing
determination of an upper limit value of the amount of spectral
surplus;
[0030] FIG. 17 is a diagram illustrating a method of describing
determination of an upper limit value of the amount of spectral
surplus;
[0031] FIG. 18 is a flowchart of a method of determining the upper
limit value of the amount of spectral surplus;
[0032] FIG. 19 is a diagram illustrating an example of a
configuration of an optical transmission apparatus according to a
third embodiment; and
[0033] FIG. 20 is a flowchart illustrating a method of controlling
output in the optical transmission apparatus in FIG. 19.
DESCRIPTION OF EMBODIMENTS
[0034] In the following, embodiments will be described with
reference to the drawings.
[0035] FIGS. 1A-1B are diagrams illustrating a technical problem to
be solved in optical transmission, found by the inventors. In FIG.
1A, multiple wavelength signals 102-1 to 102-4 are placed densely,
to be transmitted as a multiplexed optical signal 101. The
wavelength signals 102-1 to 102-4 are, for example, Nyquist pulse
signals in a limited band, or subcarrier signals used in CO-OFDM
communication. In the following description, an individual signal
that is optically multiplexed, such as a Nyquist pulse signal and a
subcarrier signal, will be referred to as a "wavelength
signal".
[0036] Each of the wavelength signals 102-1 to 102-4 has its gain
or intensity level adjusted individually so as to maintain
transmission quality of the signal favorably, and then, is output
on an optical transmission line. For example, if signal levels of
the wavelength signals 102-1 to 102-4 input into a variable
attenuator are at levels designated by dashed lines, the output
levels to the optical transmission line are adjusted for the
respective wavelength signals to form a spectrum shape designated
by solid lines. In the example in FIG. 1A, the output levels of the
wavelength signals 102-1 and 102-4 at both ends are adjusted to be
lower than the output levels of the wavelength signals 102-2 and
102-3 at the center. This control reduces interference in the
wavelength signals 102-2 and 102-3 at the center by the wavelength
signal 102-1 and 102-4 at both ends, and makes uniform the signal
quality of the multiplexed optical signal 101 as a whole. By
adjusting the amount of attenuation for each of the multiplexed
wavelength signals, it is possible to prevent degradation of
transmission quality.
[0037] However, when controlling the amount of attenuation for each
wavelength signal by using a variable attenuator or a wavelength
selective switch, controllability is limited with respect to
granularity of wavelengths. In FIG. 1A, granularity of controllable
wavelengths or variable attenuation bands where amounts of
attenuation are controllable are represented by slots 103. The slot
width expected to be used practically at the present stage is about
6.25 GHz.
[0038] In each boundary part between wavelength signals adjacent to
each other, namely, in each slot 103A including two components of
the wavelength signals, the signal quality degrades due to
deficiency and excess of the amount of attenuation. As the amount
of attenuation to achieve a gain to be obtained, a single value is
used for each of the slots 103. When adjusting gains of two
components of the wavelength signals by using this single value, it
is difficult to optimize the adjacent two wavelength signals at the
same time.
[0039] Consider a case where a target value T1 is set for
controlling output of the wavelength signal 102-3, a target value
T2 is set for controlling output of the wavelength signal 102-4,
and the intermediate value is used in the corresponding slot 103A
to be controlled.
[0040] In this case, as illustrated in FIG. 1B, the target gain
cannot be obtained at an edge of the wavelength signal 102-3, and
spectral narrowing (passband narrowing (PBN)) is generated. The
spectral narrowing is represented by a region A designated with
slant lines. The spectral narrowing corresponds to excess of the
amount of attenuation, which degrades the signal quality.
[0041] On the other hand, at an edge of the wavelength signal
102-4, the gain is higher than the target value, and spectral
surplus is generated. The spectral surplus is represented by a
region B designated with slant lines. The spectral surplus
corresponds to deficiency of the amount of attenuation, which also
degrades the signal quality.
[0042] FIG. 2 is a diagram illustrating a basic concept to solve
the technical problem illustrated in FIG. 1. Here, the output
spectrum of the variable attenuator is monitored, and based on the
target value and the monitored value (measured value), the gain in
a variable attenuation band is to be controlled, namely, the amount
of attenuation is controlled. The configuration and the method
according to the embodiment is especially effective for a case
where a slot (a variable attenuation band) 103, which is a minimum
unit for controlling output of a variable attenuator, includes two
adjacent components of the wavelength signals.
[0043] If the slot 103 to be controlled includes two adjacent
components of the wavelength signals, a control method considered
in general may be to control the sum of the amount of spectral
narrowing (the slant lined region A in FIG. 1B) and the amount of
spectral surplus (the slant lined region B in FIG. 1B) to be
minimized. If such control of minimizing the sum of the amount of
spectral narrowing and the amount of spectral surplus is executed,
the spectrum of the wavelength signal 102.sub.j and the wavelength
signal 102.sub.j+1 exhibits a shape designated by dashed lines
104.
[0044] In contrast, the control according to the embodiment pays
attention to degradation of signal quality by spectral narrowing,
and prioritizes compensation for spectral narrowing over spectral
surplus, and allows spectral surplus within a certain range.
Specifically, an upper limit value is set as a permissible amount
of spectral surplus, and the spectral narrowing is minimized within
a range in which the amount of spectral surplus does not exceed the
upper limit value.
[0045] By executing such control, the spectrum of the wavelength
signal 102.sub.j and the wavelength signal 102.sub.j+1 exhibits a
shape designated by solid lines 105. The target value T1 is
obtained for the wavelength signal 102.sub.j as the output level to
the optical transmission line. The amount of spectral surplus of
the wavelength signal 102.sub.j+1 becomes greater on the edge of
the side adjacent to the wavelength signal 102-3.
[0046] Even if spectral surplus is generated within a predetermined
range, the spectral surplus can be compensated for by adaptive
equalization in digital signal processing on the reception side. On
the other hand, compensation on the reception side is difficult for
the spectrum having narrowing (defect) generated. This is because
noise is also amplified by adaptive equalization on the reception
side, and hence, degradation of the signal quality is inevitable
for a part having the narrowing generated.
[0047] Therefore, the control is executed for an optical signal to
be output to an optical transmission line, to minimize spectral
narrowing within a range in which the amount of spectral surplus
does not exceed the permissible upper limit value. By this control,
it is possible to prevent degradation of transmission quality even
if a slot (a variable attenuation band), which is a minimum unit
for controlling output of a variable attenuator, includes two
adjacent components of the wavelength signals. In the following,
specific embodiments will be described.
First Embodiment
[0048] FIG. 3 is a schematic view of an optical transmission system
1 according to a first embodiment. The optical transmission system
1 includes multiple optical transmission apparatuses 2 connected by
optical transmission lines 4 such as optical fibers, and a network
control apparatus 3 that manages the optical transmission
apparatuses 2. A multiplexed optical signal having multiple
wavelength signals multiplexed is transmitted in the optical
transmission system 1.
[0049] FIG. 4 is a basic configuration diagram of the optical
transmission apparatus 2. FIG. 4 illustrates only minimum elements
for describing the control technology according to the embodiment.
Therefore, optical amplifiers and the like are omitted, which may
be placed at an input terminal from the optical transmission line 4
and an output terminal to the optical transmission line 4. The
optical transmission apparatus 2 may be used as a transmission
apparatus for relaying to relay a signal to a next node, or may be
used as an optical add/drop apparatus to insert (add) and branch
(drop) a signal.
[0050] The optical transmission apparatus 2 includes a variable
attenuator 21, an optical branch part 23, a spectrum monitor 25, a
memory 26, a control value calculation unit 31, and an output
control unit 32. The variable attenuator 21 has a function of
wavelength spectral separation and a function of adjusting the
amount of attenuation, to adjust the gain, namely, the amount of
attenuation of each wavelength signal included in a multiplexed
optical signal received from the optical transmission line 4, and
to output the adjusted signal. Wavelength spectral separation in
the variable attenuator 21 is implemented by a spectral separation
unit such as a refractive-index-varied array waveguide and a
diffracting grating. Adjustment of the amount of attenuation may be
an analog adjustment by the voltage, or may be a digital adjustment
controlling the coupling ratio by using a micro mirror and the
like.
[0051] A part of an optical signal output from the variable
attenuator 21 is branched by the optical branch part 23 such as a
photocoupler, and input into the spectrum monitor 25. The spectrum
monitor 25 is constituted with, for example, a variable wavelength
filter, a photodetector, and an analog-digital converter, as will
be described later. The spectrum monitor 25 measures the spectrum
of an optical signal with a predetermined resolution, and outputs
the measured result as a digitally-sampled electric signal. The
output of the spectrum monitor 25 is connected with the input of
the control value calculation unit 31.
[0052] The memory 26 stores target values for output control set
for respective wavelength signals. The target value stored in the
memory 26 is read out appropriately by the control value
calculation unit 31. The memory 26 also stores the upper limit
value of a permissible amount of spectral surplus. The upper limit
value of the amount of spectral surplus is a value determined
depending on the performance of an optical transmission apparatus
on the reception side, and measured and determined, for example,
when the network is activated. A method of determining the upper
limit value will be described later.
[0053] The control value calculation unit 31 calculates an amount
of spectral narrowing and an amount of spectral surplus, based on a
measured value obtained by the spectrum monitor 25, and a target
value read out from the memory 26. If two adjacent components of
the wavelength signals are included in a slot 103 to be controlled,
both spectral narrowing and spectral surplus may be generated. The
control value calculation unit 31 outputs the amount of spectral
narrowing and the amount of spectral surplus to the output control
unit 32 as control values. The output control unit 32 controls the
amount of attenuation, namely, the output value of the variable
attenuator 21, based on the control values calculated by the
control value calculation unit 31.
[0054] The control value calculation unit 31 and the output control
unit 32 may be implemented by discrete analog circuits or digital
circuits, or may be implemented by a single microprocessor 30.
Alternatively, the memory 26, the control value calculation unit
31, and the output control unit 32 may be implemented by a single
integrated circuit such as an ASIC (Application Specific Integrated
Circuit).
[0055] <Controlling Output Value of Variable Attenuator>
[0056] With reference to FIG. 5 and FIG. 6, calculation of the
amount of spectral narrowing and control of the amount of
attenuation will be described. In FIG. 5A, a point designated by a
black circle represents a monitored value (S.sub.out) output from
the spectrum monitor 25, and a point designated by a white circle
represents a target value (S.sub.target). Here,
S.sub.target(.lamda.) represents a target value of the spectrum at
a monitored wavelength .lamda. monitored by the spectrum monitor
25. S.sub.out(.lamda., .alpha..sub.ATT) represents a monitored
value of the spectrum at the wavelength .lamda. when the amount of
attenuation is a .alpha..sub.ATT.
[0057] S.sub.target(.lamda.): a target value of a spectrum at a
wavelength .lamda.
[0058] S.sub.out(.lamda., .alpha..sub.ATT): a monitored value of
the spectrum at a wavelength A when the amount of attenuation is
.alpha..sub.ATT
[0059] Representing the difference between the monitored value and
target value at the wavelength .lamda. when the amount of
attenuation is .alpha..sub.ATT by X(.lamda., .alpha..sub.ATT), the
difference X(.lamda., .alpha..sub.ATT) is represented by the
following Formula (1).
X ( .lamda. , .alpha. ATT ) = S out ( .lamda. , .alpha. ATT ) S
target ( .lamda. ) - 1 ( 1 ) ##EQU00001##
[0060] For example, if the monitored value S.sub.out is 0.8, and
the target value is 1, the difference X is -0.2. In this case, as
represented by U.sub.PBN in FIG. 5B, narrowing is generated in the
spectrum. The horizontal axis in FIG. 5B represents the wavelength
.lamda., and the vertical axis represents the difference X
represented by [(S.sub.out/S.sub.target)-1]. If the difference X is
represented as a plus value, spectral surplus U.sub.over is
generated.
[0061] An integral value of the amount of spectral narrowing when
the amount of attenuation is .alpha..sub.ATT is represented by
Formula (2).
U PBN ( .alpha. ATT ) = .lamda. ( 1 - S out ( .lamda. , .alpha. ATT
) S target ( .lamda. ) ) if S out ( .lamda. , .alpha. ATT ) < S
target ( .lamda. ) ( 2 ) ##EQU00002##
[0062] In Formula (2), the "if" clause represents a condition that
integration is calculated if S.sub.out(.lamda., .alpha..sub.ATT)
<S.sub.target(.lamda.) namely, a condition that integration is
calculated only for a part where the monitored value is below the
target value.
[0063] The total amount of the difference (absolute value) between
the monitored value and the target value at the wavelength .lamda.
when the amount of attenuation is .alpha..sub.ATT is represented by
Formula (3).
U error ( .alpha. ATT ) = .lamda. S out ( .lamda. , .alpha. ATT ) S
target ( .lamda. ) - 1 ( 3 ) ##EQU00003##
[0064] This U.sub.error(.alpha..sub.ATT) corresponds to the sum of
areas of U.sub.PBN and U.sub.over in FIG. 5B. Therefore, the
integral value U.sub.over(.alpha..sub.ATT) of the amount of
spectral surplus is obtained by Formula (4) as a value subtracting
U.sub.PBN(.alpha..sub.ATT) from U.sub.error(.alpha..sub.ATT).
U.sub.over(.alpha..sub.ATT)=U.sub.error(.alpha..sub.ATT)-U.sub.PBN(.alph-
a..sub.ATT) (4)
[0065] FIG. 6A illustrates a relationship among U.sub.PBN,
U.sub.over, and U.sub.error. General output control may select an
amount of attenuation that minimizes U.sub.error as the total
amount of the difference between the monitored value and the target
value (the amount of attenuation at a point P2 on the horizontal
axis). In contrast, in the embodiment, the amount of attenuation is
determined that minimizes the amount of spectral narrowing
U.sub.PBN within a range in which the amount of spectral surplus
does not exceed the upper limit (the amount of attenuation at a
point P1 on the horizontal axis).
[0066] FIG. 6B illustrates a relationship between the amount of
attenuation .alpha..sub.ATT of the variable attenuator 21 and the
signal quality (for example, bit error rate BER). Spectral surplus
can be compensated for to a certain extent by adaptive equalization
and the like on the reception side, and hence, has smaller
influence on the signal quality. On the other hand, spectral
narrowing corelates with the signal quality. The greater the amount
of attenuation .alpha..sub.ATT becomes, namely, the greater the
amount of narrowing becomes, the greater the BER is, and the more
the signal quality degrades. Thereupon, control is executed to
minimize the amount of spectral narrowing U.sub.PBN as least as
within a permissible range of spectral surplus.
[0067] Based on the amount of spectral narrowing U.sub.PBN and the
amount of spectral surplus U.sub.over calculated by the control
value calculation unit 31, the output control unit 32 of the
optical transmission apparatus 2 controls the output of the
variable attenuator 21 so as to minimize the amount of spectral
narrowing U.sub.PBN, and not to have the amount of spectral surplus
U.sub.over exceed the permissible upper limit value.
[0068] FIG. 7A is a flowchart of a method of controlling output of
an optical signal in the optical transmission apparatus 2. The
spectrum monitor 25 measures the output spectrum of the variable
attenuator 21 (Step S11). The control value calculation unit 31
compares the measured value by the spectrum monitor 25 with a
target value read from the memory 26 for each slot 103 to be
controlled, and calculates the amount of spectral
U.sub.over(.alpha..sub.ATT) by Formulas (1) to (4) described above
(Step S12). If two adjacent wavelength signals are included in the
slot to be controlled, both spectral narrowing and spectral surplus
may be generated.
[0069] The output control unit 32 changes the output value of the
variable attenuator 21 or the amount of attenuation .alpha..sub.ATT
to a value that minimizes the amount of spectral narrowing
U.sub.PBN(.alpha..sub.ATT) (Step S13). Further, the output control
unit 32 determines whether the amount of spectral surplus
U.sub.over(.alpha..sub.ATT) when the amount of spectral narrowing
U.sub.PBN(.alpha..sub.ATT) is minimized, is less than or equal to
the upper limit value (Step S14). If the amount of spectral surplus
U.sub.over(.alpha..sub.ATT) is less than or equal to the upper
limit value (YES at Step S14), the amount of attenuation
.alpha..sub.ATT set at Step S13 in the variable attenuator 21 is
maintained. If the amount of spectral surplus
U.sub.over(.alpha..sub.ATT) exceeds the upper limit (NO at Step
S14), the output control unit 32 changes the output value of the
slot to be controlled or the amount of attenuation of the variable
attenuator 21 so that the amount of spectral surplus
U.sub.over(.alpha..sub.ATT) takes the upper limit value (Step
S15).
[0070] Having completed Steps S12 to S15, the output control unit
32 determines whether there is a slot (a variable attenuation band)
yet to be controlled (Step S16), and repeats Steps S12 to S15 until
the output of all slots are controlled. This process flow may be
executed before starting communication, or may be executed during
the operation to maintain the transmission quality.
[0071] The output control according to the embodiment is especially
effective if two adjacent components of the wavelength signals are
included in a slot to be controlled of the variable attenuator 21.
Even if both spectral narrowing and spectral surplus are generated
in the slot being the minimum unit of variable attenuation control,
control for minimizing the spectral narrowing is prioritized as
long as the amount of spectral surplus does not exceed the upper
limit value. Thus, the transmission quality of the signal is
maintained.
[0072] If only a single wavelength signal is included in the slot
to be controlled, S.sub.out is controlled to be equivalent to the
target value S.sub.target. Alternatively, Steps S12 to S15 may be
executed only for slots in which two components of the wavelength
signals are included.
[0073] FIG. 7B is a flowchart that includes a step for determining
whether only a single wavelength signal is included in a slot. The
same steps as in FIG. 7A are assigned the same reference symbols,
and their description is omitted. After having measured the
spectrum at Step S11, the output control unit 32 determines whether
only a single wavelength signal is included in the slot (Step
S101). If only a single wavelength signal is included (YES at Step
S101), the output control unit 32 controls S.sub.out to be
equivalent to the target value S.sub.target, and goes forward to
Step S16.
[0074] Steps S12 to S15 and Step S102 may be executed after
determination at Step S101 has been executed for all slots.
[0075] By this method, the output of the variable attenuator 21 is
controlled so that the amount of spectral narrowing is minimized
within a range where the amount of spectral surplus is contained
below the upper limit value.
Second Embodiment
[0076] In the first embodiment, it is assumed that every optical
transmission apparatus 2 included in the optical transmission
system 1 has a spectrum monitor 25, to describe an example where
the output level to the optical transmission line 4 is optimized
for a single node (the optical transmission apparatus 2). However,
it is not always true that every optical node in a network has a
spectral monitor 25. Thereupon, in a second embodiment, output
control will be described in a case where a network includes an
optical node not having the spectrum monitor 25.
[0077] FIG. 8 is a schematic view of an optical transmission system
1A that includes an optical node 200 not having a spectral monitor
25. In the example in FIG. 8, a node 2 and a node 5 are optical
transmission apparatuses 2-1 and 2-2 that have spectrum monitors
25, respectively. Nodes 1, 3, 4, 6, and 7 are optical nodes 200 not
having a spectrum monitor 25. The optical transmission apparatus
2-1 and 2-2 execute output control, assuming existence of optical
nodes 200 not having a spectrum monitor 25.
[0078] As the control assuming existence of optical nodes 200 not
having a spectrum monitor 25, the following cases will be
described:
(1) controlling for a case where no optical transmission apparatus
2 having a spectrum monitor 25 exists as a node at a succeeding
stage (Control (1)); and (2) controlling for a case where an
optical node 200 not having a spectrum monitor 25 is interposed
between optical transmission apparatuses 2 having respective
spectrum monitors 25 (Control (2)).
[0079] FIGS. 9 to 11 are diagrams illustrating Control (1). In FIG.
9, consider a case where a multiplexed optical signal is
transmitted from an optical transmission apparatus 2-2 having a
spectral monitor 25, to an optical node 200 not having a spectral
monitor 25.
[0080] The optical transmission apparatus 2-2 has the same
configuration as the optical transmission apparatus 2 in FIG. 4.
The variable attenuator 21 is a wavelength-selective variable
attenuator, for which, for example, a wavelength selective switch
(WSS) having a function to adjust the amount of attenuation may be
used. The wavelength-selective variable attenuator 21 may control,
for example, the coupling ratio for coupling light at each
wavelength with a corresponding input/output port, to adjust the
output level.
[0081] In the optical node 200 not having a spectrum monitor 25,
the control unit 232 adjusts the output of the variable attenuator
21 level or the amount of attenuation, to match with a target value
stored in the memory 226. In the optical node 200, even if two
adjacent components of the wavelength signals are included in a
slot to be controlled, control is executed to match with a single
target value. Therefore, spectral narrowing may be generated at a
boundary part of adjacent wavelength signals.
[0082] FIG. 10 is a schematic view illustrating output control
executed in the optical transmission apparatus 2-2 in FIG. 9. Two
adjacent wavelength signals 102.sub.j and wavelength signal
102.sub.j+1 are included in a slot 103A to be controlled. Solid
lines 105 represent an output spectrum that is obtained by spectral
optimization in a single node according to the first embodiment.
Bold lines 106 represent an output spectrum that is obtained by
Control (1) by the optical transmission apparatus 2-2 according to
the second embodiment. The bold lines 106 exhibit spectral surplus
generated both in the wavelength signal 102.sub.j and the
wavelength signal 102.sub.j+1 that are adjacent to each other. The
output value of the slot to be controlled of the variable
attenuator 21 is increased until the amount of spectral surplus
reaches the upper limit.
[0083] FIG. 11 is a schematic view illustrating control of the
output of the variable attenuator 21 or the amount of attenuation.
For optimizing the level of an optical signal in a single node,
control is executed to minimize spectral narrowing within a range
in which the amount of spectral surplus does not exceed the upper
limit. Therefore, the amount of attenuation .alpha..sub.ATT of a
slot to be controlled of the variable attenuator 21 is controlled
to a point P1.
[0084] In contrast, Control (1) does not just attempt to minimize
spectral narrowing at a part where the spectral narrowing has been
generated, but generates spectral surplus up to the upper limit of
the amount of spectral surplus, to have output exceeding the target
value. The amount of attenuation .alpha..sub.ATT of the variable
attenuator 21 is set to a value at a point P3, which is further
smaller than the point P1. This control has a common point with the
output control according to the first embodiment, in terms of
making the amount of spectral narrowing zero, namely, set to the
minimum. However, to prevent spectral narrowing in advance,
spectral surplus is generated beyond a target value.
[0085] Whether the output value of a wavelength signal having
spectral narrowing generated is increased higher than the target
value, may be indicated from the network control apparatus 3 to the
optical transmission apparatus 2-2 every time the network topology
is changed or the path is switched. If two adjacent wavelength
signals are included in a slot to be controlled of the variable
attenuator 21, the optical transmission apparatus 2-2 calculates
the amount of spectral narrowing and the amount of spectral surplus
by Formula (1) to Formula (4). The optical transmission apparatus
2-2 corrects the spectral narrowing, and raises the output of the
slot to be controlled of the variable attenuator 21 until the
amount of spectral surplus reaches the upper limit. Thus, even if
the optical node 200 at the succeeding stage adjusts the amount of
attenuation for a multiplexed wavelength signal, and excess or
deficiency of the adjustment is generated, the spectral narrowing
can be checked to the minimum.
[0086] FIGS. 12 to 14 are diagrams illustrating Control (2). In
FIG. 12, an optical node 200 not having a spectral monitor 25 is
interposed between the optical transmission apparatus 2-1 and the
optical transmission apparatus 2-2 having respective spectral
monitors 25. The optical transmission apparatuses 2-1 and 2-2
execute control for compensating for spectral narrowing that may be
generated in the optical node 200 in-between, and maintaining the
amount of spectral surplus within a permissible range. The
configuration of the optical transmission apparatus 2-1 or 2-2 is
the same as the configuration of the optical transmission apparatus
2 in FIG. 4. As the variable attenuator 21, for example, a
wavelength selective switch (WSS) having a function to adjust the
amount of attenuation is used. The optical node 200 is the same as
the optical node 200 in FIG. 9.
[0087] Every time the network topology is changed or the path is
switched, the optical transmission apparatuses 2-1 and 2-2 receive
a control signal that represents whether to execute controlling the
output value of a wavelength signal having spectral narrowing
generated to be higher than a target value, from the network
control apparatus 3.
[0088] Based on the command from the network control apparatus 3,
the optical transmission apparatus 2-1 at the preceding stage
corrects the spectral narrowing, and increases the output of a slot
to be controlled of the variable attenuator 21 until the amount of
spectral surplus reaches the upper limit. When a multiplexed
optical signal having the output control applied in this way is
input into the optical transmission apparatus 2-2 at the succeeding
stage, there may be a case where the amount of spectral surplus is
over the upper limit due to influence of the performance of the
optical transmission line 4 and the optical node 200 and the like.
In such a case, based on the amount of spectral narrowing and the
amount of spectral surplus that have been calculated, the optical
transmission apparatus 2-2 weakens the output of the variable
attenuator 21 (increases the amount of attenuation) of the
apparatus itself so that the amount of spectral surplus returns to
the range within the upper limit as long as the amount of spectral
narrowing is contained in a minimum range. Thus, quality
degradation of the multiplexed optical signal output to the optical
transmission line 4 can be prevented.
[0089] FIG. 13 is a schematic view of the output control in the
optical transmission apparatus 2-2. This is similar to the output
control according to the first embodiment in that the amount of
attenuation is reduced from a point P2 at which the total
U.sub.error of the amount of spectral narrowing and the amount of
spectral surplus is minimum, to prioritize prevention of spectral
narrowing generation. Assume that the amount of attenuation
.alpha..sub.ATT of a slot to be controlled of the variable
attenuator 21 has been set to a point P4 by the previous process.
In the previous process, at the point P4, the amount of attenuation
takes the minimum, and the amount of spectral surplus takes the
upper limit value. Assume that in the current process, the amount
of spectral surplus calculated from Formulas (1) to (4) based on a
measured value and a target value exceeds the upper limit. In this
case, the optical transmission apparatus 2-2 weakens the output of
the slot to be controlled of the variable attenuator 21 until the
amount of spectral surplus returns to the upper limit.
Specifically, the optical transmission apparatus 2-2 increases the
amount of attenuation .alpha..sub.ATT of the slot to be controlled
of the variable attenuator 21 from the point P4 to a point P5.
[0090] By this process, quality degradation of the multiplexed
optical signal output to the optical transmission line 4 can be
prevented.
[0091] FIG. 14 is a flowchart illustrating the control executed in
the optical transmission apparatuses 2-1 and 2-2 included in the
optical transmission system 1A. This control flow includes a flow A
and a flow B. The flow A corresponds to a process executed in the
optical transmission apparatus 2-2 according to Control (1), or a
process in the optical transmission apparatus 2-1 at the preceding
stage according to Control (2). The flow B corresponds to a process
executed in the optical transmission apparatus 2-2 at the
succeeding stage according to Control (2). In the following
description, the optical transmission apparatus 2-1 or 2-2 may be
generally referred to as the "optical transmission apparatus
2".
[0092] The optical transmission apparatus 2 measures the output
spectrum of the variable attenuator 21, such as a WSS, by the
spectrum monitor 25 in the apparatus itself (Step S21). The control
value calculation unit 31 compares a measured value obtained by the
spectrum monitor 25 with a target value read from the memory 26,
and calculates the amount of spectral narrowing and the amount of
spectral surplus from Formulas (1) to (4) (Step S22).
[0093] Next, based on the calculated amount of spectral narrowing
and the amount of spectral surplus, the output control unit 32
changes the output value of a slot to be controlled of the variable
attenuator 21, to a value with which the amount of spectral surplus
takes the upper limit (Step S23). By this change of the output
value, the amount of spectral narrowing becomes minimum (zero). If
two adjacent components of the wavelength signals are included in
the slot to be controlled, spectral surplus may be generated in
both wavelength signals. If only a single wavelength signal is
included in the slot to be controlled of the variable attenuator
21, the output of the variable attenuator 21 is controlled to an
output level that corresponds to the upper limit value of the
amount of spectral surplus, not only for a case where the measured
value exceeds the target value, but also for a case where the
measured value is under the target value.
[0094] The process flow branches off depending on whether a node
capable of spectral monitoring exists at the succeeding stage of
the optical transmission apparatus 2 (Step S25). If a node capable
of spectral monitoring does not exist at the succeeding stage of
the optical transmission apparatus 2 (NO at Step S25), the output
control of the optical transmission apparatus 2 in the network
ends, which corresponds to Control (1) described above. The optical
node 200 at the succeeding stage (see FIG. 8) executes general
output control on the output of the variable attenuator to match
with the target value. Even for a case in which spectral narrowing
may generate at the boundary part between two adjacent wavelength
signals, degradation of transmission quality can be prevented
because the output is compensated for in advance by the optical
transmission apparatus 2 at the preceding stage.
[0095] If a node capable of spectral monitoring exists at the
succeeding stage of the optical transmission apparatus 2 (YES at
Step S25), the control flow B is executed by the optical
transmission apparatus 2 at the succeeding stage, which corresponds
to Control (2).
[0096] The optical transmission apparatus 2 at the succeeding stage
measures the output spectrum of the variable attenuator 21, such as
a WSS, by the spectrum monitor 25 in the apparatus itself (Step
S31). The control value calculation unit 31 compares a measured
value obtained by the spectrum monitor 25 with a target value read
from the memory 26, and calculates the amount of spectral narrowing
and the amount of spectral surplus from Formulas (1) to (4) (Step
S32).
[0097] If the amount of spectral surplus calculated at Step S32
exceeds the upper limit, the output control unit 32 of the optical
transmission apparatus 2 at the succeeding stage puts the amount of
spectral surplus back to the upper limit value. More specifically,
the output control unit 32 increases the amount of attenuation of
the slot to be controlled of the variable attenuator 21, and
changes the output value of the variable attenuator 21 so that the
amount of spectral narrowing becomes minimum within a range of the
upper limit value of the amount of spectral surplus (Step S33).
Since the amount of spectral narrowing can be checked to the
minimum within a range of the upper limit value of the amount of
spectral surplus, degradation of transmission quality can be
prevented.
[0098] <Determination of Upper Limit Value of Amount of Spectral
Surplus>
[0099] With reference to FIG. 15A to FIG. 18, determination of the
upper limit value of the amount of spectral surplus will be
described. Determination of the upper limit value of the amount of
spectral surplus described in the following is applied to both the
first embodiment and the second embodiment.
[0100] The upper limit value of the amount of spectral surplus is
determined by the performance (adaptive equalization capability) on
the reception side. FIG. 15A illustrates the amount of spectral
surplus U.sub.over, and the bit error rate BER, as functions of the
amount of attenuation .alpha..sub.ATT, respectively. The upper
limit of BER that can be compensated for by adaptive equalization
on the reception side is a threshold Q. Here, R represents the
upper limit value of the amount of spectral surplus U.sub.over when
the BER at the succeeding stage node takes the threshold Q. Note
that the parameter representing signal quality is not limited to
the BER, but another parameter may be used such as carrier-to-noise
ratio (CN ratio).
[0101] As illustrated in FIG. 15B, the transmission side controls
the output level (or the amount of attenuation) of the variable
attenuator 25 within a range not exceeding the upper limit of the
amount of spectral surplus U.sub.over.
[0102] FIG. 16 illustrates a configuration of an optical
transmission apparatus 2A and an optical transmission apparatus 2B
used for determination of the upper limit value of the amount of
spectral surplus. The upper limit of the amount of spectral surplus
is determined by measuring the BER along with the amount of
spectral surplus, for example, when the network is activated in a
state where the optical transmission apparatus 2A is connected with
the optical transmission apparatus 2B. For convenience's sake, FIG.
16 illustrates only those elements that are required when an
optical signal for testing is transmitted from the optical
transmission apparatus 2A to the optical transmission apparatus
2B.
[0103] The optical transmission apparatus 2A includes a
wavelength-selective variable attenuator (a WSS, etc.) 21, an
optical branch part 23, a spectrum monitor 25, a memory 26, a
control value calculation unit 31, an output control unit 32, and
in addition, a receiver 27A, a transmitter 28A, and an optical
multiplexer 29.
[0104] The optical transmission apparatus 2B includes a
wavelength-selective variable attenuator (a WSS, etc.) 21, an
optical branch part 23, a spectrum monitor 25, a memory 26, a
control value calculation unit 31, an output control unit 32, and
in addition, a receiver 27B, a transmitter 28B, and optical
multiplexer 29, and an upper limit value determination unit 33. The
control value calculation unit 31, the upper limit value
determination unit 33, and the output control unit 32 may be
implemented by, for example, a single processor 30B, or may be
implemented as discrete circuits. Also, the upper limit value
determination unit 33 may be embedded in the output control unit 32
as a part.
[0105] An optical signal for testing is output from the transmitter
28A of the optical transmission apparatus 2A. The optical signal
for testing is output to the optical transmission line 4 via the
optical multiplexer 29, and received by the optical transmission
apparatus 2B. In the optical transmission apparatus 2B, wavelength
signals are branched off by the variable attenuator 21, to be input
into the receiver 27B. A part of the optical signal is branched by
the optical branch part 23, to be measured by the spectrum monitor
25. The receiver 27B includes, for example, an error detection
circuit, and based on the output of the error detection circuit,
the BER is measured. The measured BER is stored in, for example,
the memory 26.
[0106] The control value calculation unit 31 calculates the amount
of spectral surplus based on a measured value of the spectrum
monitor 25 and a target value stored in the memory 26. The output
control unit 32 increases the output value of the variable
attenuator 21 until the BER reaches the threshold Q as the
permissible limit (decreases the amount of attenuation). While
increasing the output value of the variable attenuator 21, the
output control unit 32 controls measuring the BER and the amount of
spectral surplus. Once the BER has reached the threshold Q, the
upper limit value determination unit 33 determines the amount of
spectral surplus at that moment as the upper limit value of the
amount of spectral surplus.
[0107] By the configuration in FIG. 16, the upper limit value of
the amount of spectral surplus is determined including an influence
of the optical transmission line 4.
[0108] FIG. 17 illustrates another method of determining the upper
limit value of the amount of spectral surplus. In FIG. 17, the
optical transmission apparatus 2B by itself determines the upper
limit value of the amount of spectral surplus. For convenience's
sake, FIG. 17 illustrates only those elements that are required for
determining the upper limit value of the amount of spectral
surplus. An optical signal for testing is transmitted from the
transmitter 28B of the optical transmission apparatus 2B to the
receiver 27B of the apparatus itself via the optical multiplexer
29. Wavelength signals are branched off by the variable attenuator
21, and the BER is measured by the receiver 27B. A part of the
optical signal is branched by the optical branch part 23, to be
measured by the spectrum monitor 25.
[0109] While increasing the output value of the variable attenuator
21, the output control unit 32 controls measuring the BER and the
amount of spectral surplus. Once the BER has reached the threshold
Q, the upper limit value determination unit 33 determines the
amount of spectral surplus at that moment as the upper limit value
of the amount of spectral surplus. The determined upper limit value
of the amount of spectral surplus is used for controlling output
from the optical transmission apparatus 2A at the preceding stage
to the optical transmission line 4. By the configuration in FIG.
17, the upper limit value of the amount of spectral surplus can be
determined, without setting up reception and transmission with the
optical transmission apparatus 2A on the other end.
[0110] FIG. 18 is a flowchart of determining the upper limit value
of the amount of spectral surplus. An optical signal for testing is
transmitted from the transmitter 28A of the optical transmission
apparatus 2A in the forward direction, or from the transmitter 28B
of the optical transmission apparatus 2B in the reverse direction
(Step S41). The spectrum monitor 25 in the optical transmission
apparatus 2B measures the spectrum at the receiving terminal (Step
S42).
[0111] The control value calculation unit 31 of the optical
transmission apparatus 2B compares a target value read from the
memory 26 with the measured value, to calculate the amount of
spectral surplus (Step S43). At the same time as Step S43, or
before or after Step S43, the receiver 27B of the optical
transmission apparatus 2B measures the signal quality such as the
BER (Step S44). The upper limit value determination unit 33
determines whether the signal quality has reached a permissible
limit value (threshold Q) (Step S45). If the signal quality has
reached the permissible limit value (YES at Step S45), the upper
limit value determination unit 33 determines the amount of spectral
surplus at that moment as the upper limit value (Step S47). If the
signal quality has not reached the permissible limit value, the
output control unit 32 of the optical transmission apparatus 2B
increases the output value of the variable attenuator 21 (Step
S46), and the process goes back to Step S42. By repeating Steps S42
to S45 until the signal quality reaches the permissible limit
(threshold Q), the upper limit value of the amount of spectral
surplus is determined.
Third Embodiment
[0112] FIG. 19 illustrates an example of a configuration of an
optical transmission apparatus 2c according to a third embodiment.
The optical transmission apparatus 2C executes the output control,
and determines the upper limit value of the amount of spectral
surplus in response to control signals from a network control
apparatus 3. For convenience's sake, FIG. 19 illustrates a
configuration when an optical signal is transmitted from the left
to the right on the page of the figure.
[0113] The output control for minimizing spectral narrowing within
a range where the amount of spectral surplus does not exceed the
upper limit may be executed with using a control signal from the
network control apparatus 3 as a trigger. Also, determining the
upper limit value of the amount of spectral surplus may be
triggered by a control signal from the network control apparatus
3.
[0114] The optical transmission apparatus 2C includes a first
optical amplifier 41, a second optical amplifier 42, a first
variable attenuator 21a, a second variable attenuator 21b, a first
optical branch part 22, a second optical branch part 23, a first
spectrum monitor 24, a second spectrum monitor 25, a receiver 27,
and a transmitter 28. The second spectrum monitor 25 is mainly used
for the output control for minimizing spectral narrowing. The first
spectrum monitor 24 is mainly used for determining the upper limit
value of the amount of spectral surplus.
[0115] The optical transmission apparatus 2C also includes a target
value memory 261, a memory for measured values 262, a node control
circuit 331, a control value calculation circuit 311, and an output
control circuit 321. The target value memory 261 stores a target
value of spectrum control. The memory for measured values 262
stores a measurement result of a spectrum, a measurement result of
signal quality, the upper limit value of the amount of spectral
surplus, and the like. The node control circuit 331 may be included
in a communication interface with the network control apparatus 3
as a part. The control value calculation circuit 311 and the output
control circuit 321 are logic circuits including a comparison and
arithmetic circuit and an integral circuit.
[0116] For the output control of the variable attenuator 21b, the
node control circuit 331 receives a control signal A commanding the
output control from the network control apparatus 3. The control
signal A is transmitted to a node on a path whose transmission
quality of an optical signal is below the permissible level, for
example, based on a monitored result of the network transmission
quality by the network control apparatus 3. The control signal A
includes a command of the output control and information about
wavelengths to be measured.
[0117] In response to receiving the control signal A from the
network control apparatus 3, the node control circuit 331 supplies
a control signal that includes a command to measure the spectrum
and information about wavelengths to be measured to the spectrum
monitor 25. The supply of the control signal from the node control
circuit 331 to the spectrum monitor 25 is designated by a bold
dashed line in the figure. The spectrum monitor 25 includes, for
example, a variable wavelength filter 251, a photodetector 252, and
an analog-digital converter 253. The spectrum monitor 25 monitors
the output signal of the variable attenuator 21b based on the
control signal, and supplies a measurement result and information
about wavelengths to be measured to the control value calculation
circuit 311. A signal output to the control value calculation
circuit 311 is a digital signal sampled by the analog-digital
converter 253. A target value of the spectrum control is also input
into the control value calculation circuit 311 from the target
value memory 261.
[0118] Based on the target value and the measurement result of the
spectrum, the control value calculation circuit 311 calculates the
amount of spectral narrowing and the amount of spectral surplus
from Formulas (1) to (4). The calculated amount of spectral
narrowing and the amount of spectral surplus are stored in the
memory for measured values 262. The calculated values may be input
into the output control circuit 321. The output control circuit 321
refers to the upper limit value of the amount of spectral surplus
stored in the memory for measured values 262, and controls the
output values or the amounts of attenuation of the variable
attenuator 21a and the variable attenuator 21b so that the amount
of spectral narrowing becomes minimum and the amount of spectral
surplus does not exceed the upper limit.
[0119] Next, a case will be described in which the upper limit
value of the amount of spectral surplus is determined based on
network control. The node control circuit 331 receives a control
signal B commanding to determine the upper limit value of the
amount of spectral surplus from the network control apparatus 3. In
response to the control signal B, the node control circuit 331
supplies a control signal for determining the upper limit value to
the first spectrum monitor 24, the receiver 27, and the transmitter
28. The control signal for determining the upper limit value
includes, for example, a command to transmit an optical signal for
testing, a command to start spectral monitoring for the first
spectrum monitor 24, and a command to measure the quality of the
optical signal for testing.
[0120] The transmitter 28 transmits the optical signal for testing.
The optical signal for testing is transmitted in the reverse
direction by the variable attenuators 21b and 21a, and received by
the receiver 27. The receiver 27 measures the reception quality
(the BER, etc.) of the optical signal for testing in response to
the control signal. The measured value of BER is stored in the
memory for measured values 262.
[0121] The output control circuit 321 increases the output values
of the variable attenuators 21a and 21b until the measured values
of the BER stored in the memory for measured values 256 reach the
threshold Q as the permissible limit. The receiver 27 measures the
BER for the increased output values, and writes the BER in the
memory for measured values 262 sequentially.
[0122] The first spectrum monitor 24 starts measuring the quality
of the optical signal for testing following the control signal for
determining the upper limit value, and every time the output values
of the variable attenuators 21a and 21b are increased, measures the
quality of the optical signal for testing. The configuration of the
first spectrum monitor 24 is the same as the configuration of the
second spectrum monitor 25, including a variable wavelength filter
241, a photodetector 242, and an analog-digital converter 243. A
measurement result of the first spectrum monitor 24 is input into
the control value calculation circuit 311.
[0123] The control value calculation circuit 311 calculates the
amount of spectral surplus based on a measured value of the first
spectrum monitor 24 and a target value stored in the target value
memory 261.
[0124] The output control circuit 321 determines the amount of
spectral surplus when the measured value of the BER written in the
memory for measured values 262 reaches the threshold Q as the
permissible limit, as the upper limit value of the amount of
spectral surplus. The upper limit value is stored in the memory for
measured values 262 from the control value calculation circuit
311.
[0125] Although the control value calculation circuit 311 and the
output control circuit 321 are implemented as discrete circuits in
FIG. 19, functions of the control value calculation circuit 311 and
the output control circuit 321 may be implemented by a single
microprocessor. Alternatively, they may be implemented as a single
integrated circuit together with the target value memory 261 and
the memory for measured values 262.
[0126] FIG. 20 is a flowchart of the output control in the optical
transmission apparatus 2C in FIG. 19. The network control apparatus
3 transmits a control signal A commanding to adjust the output
value of an optical signal to the optical transmission apparatus 2C
(Step S51). In response to receiving the control signal A, the node
control circuit 331 supplies a command to measure the spectrum and
information about wavelengths to be measured to the second spectrum
monitor 25 (Step S52). The measurement result by the spectrum
monitor 25 is input into the control value calculation circuit 311
along with the measured wavelength (monitored wavelength) (Step
S53).
[0127] The target value for each wavelength signal is input into
the control value calculation circuit 311 from the target value
memory 261 (Step S54). Based on the target value and the measured
value by the spectrum monitor 25, the control value calculation
circuit 311 calculates the amount of spectral narrowing and the
amount of surplus from Formulas (1) to (4). The calculated value is
stored in the memory for measured values (Step S55).
[0128] The output control circuit 321 reads out the upper limit
value of the amount of spectral surplus and the amount of spectral
narrowing from the memory for measured values 262. Thus, the upper
limit value of the amount of spectral surplus and the amount of
spectral narrowing are supplied to the output control circuit 321
from the memory for measured values 262 (Step S56).
[0129] The output control circuit 321 determines whether the amount
of spectral narrowing is minimum (Step S57), and if minimum (YES at
Step S57), maintains the output value or the amount of attenuation
used at that moment, and ends the process. If the amount of
spectral narrowing is not minimum (NO at Step S57), the output
control circuit 321 determines whether the amount of spectral
surplus has reached the upper limit value (Step S58). If the amount
of spectral surplus has reached the upper limit value (YES at Step
S58), the output control circuit 321 determines that the amount of
spectral surplus has reached the permissible limit during the
process of minimizing the spectral narrowing, and ends the process.
In this case, the output value or the amount of attenuation when
the process ends is used.
[0130] If the amount of spectral surplus has not reached the upper
limit value (NO at Step S58), the output value can be increased
further. Therefore, the output control circuit 321 transmits a
control signal commanding to increase the output values to the
variable attenuators 21a and 21b (or to decrease the amounts of
attenuation) along with a control band (Step S59). After that, the
process goes back to Step S52, repeats Steps S52 to S58 with the
updated output values or the amounts of attenuation, and when the
amount of spectral surplus reaches the upper limit, the process
ends.
[0131] If the output of the variable attenuator has been raised to
the upper limit of the amount of spectral surplus, it is unlikely
that the spectral narrowing is still generated. However, even if
the spectral narrowing is still generated, influence on the
transmission quality is checked maximally. Thus, transmission
quality of an optical signal output to the optical transmission
line 4 can be maintained favorably.
[0132] The flow for determining the upper limit value of spectral
surplus is substantially the same as the process flow in FIG. 18
except that the node control circuit 331 receives a control signal
from the network control apparatus 3, and the illustration is
omitted.
[0133] A process for minimizing spectral narrowing based on a
control signal from the network control apparatus 3 is applicable
to the optical transmission system 1A that includes the optical
node 200 not having a spectral monitor 25 as in the second
embodiment.
[0134] All examples and conditional language recited herein are
intended for pedagogical purposes to aid the reader in
understanding the invention and the concepts contributed by the
inventor to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions, nor does the organization of such examples in the
specification relate to a showing of the superiority and
inferiority of the invention. Although the embodiments of the
present invention have been described in detail, it should be
understood that the various changes, substitutions, and alterations
could be made hereto without departing from the spirit and scope of
the invention.
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